The present invention relates to flight control systems for rotary-wing rotorcraft and more particularly to a flight control system which provides a low pilot-workload trim attitude control.
Demand for low pilot-workload helicopters continues to increase. With the establishment of ADS-33 as the dominant handling qualities specification, the rotorcraft industry has been pursuing various methods to provide advanced flight control features. One primary method of compliance with ADS-33 is employment of a Fly-By-Wire (FBW) flight control system to decouple pilot inputs and rotorcraft dynamics.
Conventional FBW systems have implemented an Attitude Command/Velocity Hold (ACVH) response type in which rotorcraft attitude is directly proportional to the cyclic controller displacement from center. ACVH mode is required by the ADS-33 specification to provide reduced pilot workload especially in a degraded visual environment (DVE). Once the cyclic controller is returned to a neutral position (center detent for unique trim cyclic), the flight control system acquires a new velocity reference and engages the velocity hold part of ACVH.
One difficulty associated with ACVH presents itself in determining what attitude should be attained when the controller is returned to the neutral position. Since the velocity hold feature of ACVH is activated when the controller is returned to neutral position, it is desirable to have the rotorcraft return to the natural “trim” attitude. The difficulty appears when determining this “trim” attitude, since rotorcraft center of gravity (CG) has a direct impact on the “trim” attitude for a given flight condition.
In the past, ACVH has only been successfully implemented on relatively small scout/attack type rotorcraft which have a narrow CG range. Because of this narrow CG range, computing trim attitude for a given flight condition was implemented through a simple look-up table that provides trim attitude as a function of airspeed. Attempts to implement ACVH on larger transport/utility category rotorcraft, such estimation of the trim attitude, is impossible due to the large range of usable rotorcraft CG. In hover, for example, trim attitude may vary from 3 degrees nose up to 9 degrees nose up, depending on the current rotorcraft cargo load, fuel load and stabilator position.
Use of a look-up table that provides a “nominal” estimate also does not correct this difficulty as carrying any attitude error when the pilot is attempting to bring the rotorcraft into a hover may result in undesirable velocity transients. As the pilot stabilizes the rotorcraft in or near hover, the cyclic controller typically remains outside of detent due to the trim attitude estimation error from the “nominal” attitude. As the pilot releases the cyclic controller to detent, the rotorcraft attitude then changes by a few degrees—since any motion of the cyclic controller is interpreted by the flight control system as an attitude command—and the rotorcraft inevitably accelerates. After a few seconds, the velocity hold feature activates and the rotorcraft recovers hover, however, pilots have deemed this behavior unacceptable.
Accordingly, it is desirable to provide an algorithm for a flight control system that meets the ADS-33 specification to determine what attitude should be attained when a cyclic controller is returned to the neutral position for a rotorcraft with a large range of usable rotorcraft CG without airspeed specific look-up tables, or other estimation devices to provide reduced pilot workload.